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I'm just doing some reading, and it mentioned capping the fermenter to carbonate the beer by trapping the remaining co2. Has anyone given it a go? What were the results?

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I would recommend to read this article (byo.com/stories/issue/item/…) on fermenting in a corny keg. It describes necessary procedures and equipment, and also explains pros and cons. – Sander Mar 10 '14 at 12:53
It's a very uncertain way to control carbonation and there's really nothing to be gained by it in terms of beer quality. – Denny Conn Mar 10 '14 at 15:31

Glass carboys are not rated for pressure, I would definitely not recommend trying it there.

If fermenting or finishing in a metal vessel (like a corny keg), you can use a spunding valve to control the amount of pressure in the keg to force carbonation, similar to actively adding CO₂ to the keg to force carbonate after fermentation.

It's a practice born out of the Rhineheitsghebot about not using sugar to carbonate. Modern practices make it easier to carbonate with no ill effect on the beer, as Denny mentions.

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